The present invention concerns a microwave dielectric ceramic composition and, more specifically, it relates to a microwave dielectric ceramic composition having a high relative dielectric constant (hereinafter referred to as ".epsilon.r") while maintaining non-lead Q factor (hereinafter referred to as "Qu") and temperature coefficient of resonance frequency (hereinafter referred to as ".tau.f") each in a range of practical characteristic, as well as preparing method therefor.
The present invention is applicable, for example, to dielectric resonators in a microwave, microwave integrated circuit substrates and impedance matching of various kinds of microwave circuits.
Since the microwave dielectric ceramic composition (hereinafter referred to as a "ceramic composition") has a dielectric loss tending to increase as the working frequency becomes higher, it has been desired for a ceramic composition of large Qu in a microwave frequency region.
As dielectric ceramic materials in the prior art, there have been known, for example, a ceramic composition in which a crystal structure has two phases of a perovskite phase and an ilumenite phase (Japanese Patent Laid-Open Hei 2-129065) and a ceramic composition comprising MgTiO.sub.3 and TiO.sub.2 incorporated with a predetermined amount of CaTiO.sub.3 (Japanese Patent Laid-Open Sho 52-118599).
However, the former ceramic composition contain other ingredients such as Nd.sub.2 O.sub.3, PbO and ZnO in a great amount and, in addition, the value for Qu is not always high. The latter ceramic composition involves a problem that .tau.f varies greatly as from +87 to -100 ppm/.degree.C. in a mixing range of CaTiO.sub.3 from 3 to 10% by weight and it is difficult to control it to a smaller value nearly equal to zero.